Title

Author

Date of Award

1986

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

F.D. Holland, Jr

Abstract

Fourteen cores were sampled and processed, and conodonts obtained were used in the biozonation of the Bakken and basal Lodgepole Formations in the Williston Basin in North Dakota. The Bakken Formation of Devonian and Mississippian age and the Lodgepole Formation, entirely of Mississippian age, are subsurface units in North Dakota. The Bakken Formation (44 metres thick) consists of, in ascending order: a lower, dark, organic shale; a middle calcareous siltstone that is divisible into three lithic units; and an upper, dark, indurated, organic shale. The Lodgepole Formation (160 to 260 metres thick) is an argillaceous, fossiliferous limestone near the Basin center that varies to oolitic and skeletal limestone near the Basin margins.

Conodonts were recovered from the three members of the Bakken, by oxidizing in household bleach, and from the basal 15.5 metres of the Lodgepole, by treatment in acetic acid, in 10 of the 14 cores sampled. The 1963 discrete elements recovered were placed in 95 species-level form-taxa assignable to 22 form-genera: 15 percent (289) from the lower member of the Bakken, less than 1 percent (3) from the middle member of the Bakken, 47 percent (930) from the upper member of the Bakken, and 37 percent (741) from the basal Lodgepole. Twenty-five Bakken Formation form-taxa and all 61 Lodgepole Formation form-taxa are reported for the first time. Three-fourths (919) of the Bakken specimens were obtained from three, separate, thin, lag zones, one in the lower member of the Bakken and two in the upper member of the Bakken. No lag zones were observed in the Lodgepole. Lags may have resulted from accumulation of coarse-grained fossil and inorganic debris under sediment-starved conditions, winnowing out of fine-grained elastics and organic material during deposition that concentrated coarse-grained fossils and inorganic material, and selective winnowing of partially lithified sediment by evanescent currents. Lags are important stratigraphically but are of little value in recognizing specific conodont biozones.

At least one Famennian (Lower Palmatolepis gracilis expansa) and possibly five Kinderhookian (siphonodellid) conodont biozones were recognized in the Bakken and basal Lodgepole Formations. Thirty-eight biostratigraphically useful form-species were used for recognition of the biozones. Form-genera recovered include, in order of decreasing abundance (species-level form-taxa listed in parentheses): Siphonodella (9), Polygnathus (9), Pseudopolygnathus (5), "Spathognathodus" (5), Palmatolepis (3), Bispathodus (3), Dinodus (2), Branmehla (1), and Elictognathus (1). Conodonts obtained from the lower member of the Bakken suggest the presence of the palmatolepid-polygnathid biofacies indicative of an outer shelf environment.

Siphonodella sulcata, from the upper member of the Bakken, suggests that this member may be as old as earliest Kinderhookian, but because of its association in lag zones with conodonts representative of younger biozones, its stratigraphic significance is diminished. Due to the paucity of conodonts in the middle member, further refinement of the Devonian-Mississippian systemic boundary was not possible. The conodont fauna supports the concept of continuous sedimentation from the Bakken Formation into the Lodgepole Formation.